Substrates with electronically active components distributed over the extent of the substrate may be used in a variety of electronic systems, for example, flat-panel imaging devices such as flat-panel liquid crystal or organic light emitting diode (OLED) display devices and in flat-panel solar cells. A variety of methods may be used to distribute electronically active circuits over substrates, including forming the electronically active circuits on a substrate and forming the components on separate substrates and placing them on a substrate. In the latter case, a variety of assembly technologies for device packaging may be used.
The electronically active components are typically formed on a flat-panel substrate by sputtering a layer of inorganic semiconductor material or by spin-coating organic material over the entire substrate. Inorganic semiconductor materials can be processed to improve their electronic characteristics, for example amorphous silicon can be treated to form low-temperature or high-temperature poly-crystalline silicon. In other process methods, microcrystalline semiconductor layers can be formed by using an underlying seeding layer. These methods typically improve the electron mobility of the semiconductor layer. The substrate and layer of semiconductor material can be photolithographically processed to define electronically active components, such as transistors. Such transistors are known as thin-film transistors (TFTs) since they are formed in a thin layer of semiconductor material, typically silicon. Transistors may also be formed in thin layers of organic materials. In these devices, the substrate is often made of glass, for example Corning Eagle® or Jade® glass designed for display applications.
The above techniques have some limitations. Despite processing methods used to improve the performance of thin-film transistors, such transistors may provide performance that is lower than the performance of other integrated circuits formed in mono-crystalline semiconductor material. Semiconductor material and active components can be provided only on portions of the substrate, leading to wasted material and increased material and processing costs. The choice of substrate materials can also be limited by the processing steps necessary to process the semiconductor material and the photo-lithographic steps used to pattern the active components. For example, plastic substrates have a limited chemical and heat tolerance and do not readily survive photo-lithographic processing. Furthermore, the manufacturing equipment used to process large substrates with thin-film circuitry is relatively expensive. Other substrate materials that may be used include quartz, for example, for integrated circuits using silicon-on-insulator structures as described in U.S. Patent Publication No. 2010/0289115 and U.S. Patent Publication No. 2010/0123134. However, such substrate materials can be more expensive or difficult to process.
In further manufacturing techniques, a mono-crystalline semiconductor wafer is employed as the substrate. While this approach can provide substrates with the same performance as integrated circuits, the size of such substrates may be limited, for example, to a 12-inch diameter circle, the wafers are relatively expensive compared to other substrate materials such as glass, polymer, or quartz, and the wafers are rigid.
A variety of other methods are used for distributing electronically functional components over a substrate in the circuit board assembly industry include, for example, pick-and-place technologies for integrated circuits provided in a variety of packages such as pin-grid arrays, ball-grid arrays, and flip-chips. However, these techniques may be limited in the size of the integrated circuits that can be placed so that the integrated circuits and their packaging can be larger than is desired.
Another method for transferring active components from one substrate to another is described in U.S. Pat. No. 7,943,491. In this approach, small integrated circuits are formed on a semiconductor wafer. The small integrated circuits, or chiplets, are released from the wafer by etching a layer formed beneath the circuits. A PDMS stamp is pressed against the wafer and the process side of the chiplets is adhered to the stamp. The chiplets are pressed against a destination substrate or backplane and adhered to the destination substrate. In another example, U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate or backplane.
In some cases, however, not all of the elements are transferred from the wafer to the destination substrate by the stamp, for example, due to process abnormalities or undesired particles on the stamp, the wafer, or the destination substrate. It is also possible that the elements themselves are defective due to materials or manufacturing process errors in the wafer. Such problems can reduce manufacturing yields, increase product costs, and necessitate expensive repair or rework operations.
The electrical connections between the small integrated circuits and the backplane contact pads are typically made by photolithographic processes in which a metal is evaporated or sputtered onto the small integrated circuits and the destination substrate to form a metal layer; the metal layer is coated with a photoresist that is exposed to a circuit connection pattern and the metal layer and photoresist are developed by etching and washing to form the patterned electrical connections between the small integrated circuits and the connection pads on the destination substrate. Additional layers, such as interlayer dielectric insulators, can also be required. This process is expensive and requires a number of manufacturing steps. Moreover, the topographical structure of the small integrated circuits over the destination substrate renders the electrical connections problematic; for example it can be difficult to form a continuous conductor from the destination substrate to the small integrated circuit because of the differences in height over the surface between the small integrated circuits and the destination substrate.
Some electronic systems, such as displays, use arrays of devices that are typically controlled with either a passive-matrix (PM) control employing electronic circuitry external to the display substrate or an active-matrix (AM) control employing electronic circuitry formed directly on the display substrate and storing data associated with each light-emitting element. Both OLED displays and LCDs using passive-matrix control and active-matrix control are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066.
Typically, in an active-matrix-controlled display, each display sub-pixel is controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix OLED display, each control element includes two transistors (a select transistor and a drive transistor) and one capacitor for storing a charge specifying the desired luminance of the sub-pixel. Each OLED element employs an independent control electrode connected to the power transistor and a common electrode. In contrast, an LCD typically uses a single-transistor circuit. Control of the light-emitting elements is usually provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
Active-matrix circuitry is commonly achieved by forming thin-film transistors (TFTs) in a semiconductor layer formed on a display substrate and employing a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon or poly-crystalline silicon and is distributed over the entire flat-panel display substrate. The semiconductor layer is photolithographically processed to form electronic control elements, such as transistors and capacitors. Additional layers, including insulating dielectric layers and conductive metal layers, are provided, often by evaporation or sputtering, and photolithographically patterned to form electrical interconnections, structures, or wires.
Surface-mount devices (SMDs) are an alternative way to provide electrical elements on a substrate or backplane. Such devices, as their name suggests, include electrical connections that are typically placed on the surface and in contact with a backplane rather than including pins that extend through vias in the backplane. Surface-mount technology (SMT) is widely used in the electronics industry to provide high-density printed-circuit boards (PCBs). In particular, a well-developed and inexpensive infrastructure exists for making and integrating two-terminal surface-mount devices, such as resistors or capacitors, into printed circuit boards. However, the smallest surface-mount device readily available is several hundred microns long and wide, precluding their use for applications requiring integrated circuits with circuit elements having a size of several microns, or less.
There is a need, therefore, for structures and methods that enable the electrical interconnection of small integrated circuits, such as micro transfer printed chiplets, to destination substrates and that provide a matrix-addressed system with small high-resolution elements that is tolerant of manufacturing and materials variability and particle contamination and enables repair.